The activities of the Dean of Canterbury and a prominent folk singer were closely monitored for decades by MI5, hitherto secret files released yesterday at the National Archives show.

Hewlett Johnson, known as the Red Dean, supported the Republicans against Franco during the Spanish civil war, and became friendly with the philosopher Bertrand Russell. The two men strongly opposed nuclear weapons - at one time a large sign on the front of the deanery said: "Christians Ban Nuclear Weapons".

An MI5 officer, tipped off about Johnson's travel plans - before he was appointed Dean - noted in 1921 that he had "no information at all about the Rev Canon Hewlett Johnson except that he is a subscriber to the National Council of Civil Liberties [now Liberty]".

The MI5 man added: "I do not think this fact and his connection with Bertrand Russell would be sufficient grounds for refusing him permission to go to Vienna via Germany."

MI5 recorded that in 1938 Johnson was criticising "racial persecution in Germany". But the files show that MI5 appeared particularly concerned about his potential influence during the war. One file reveals the suspicious approach to communist sympathisers of the security services. In a memo dated July 11 1940 MI5 noted: "We have unimpeachable evidence that the Dean of Canterbury has recently subscribed to the Communist party programme."

Three years later an MI5 officer wrote to headquarters of the south-eastern command of the home forces: "It is agreed that it is undesirable for the Dean of Canterbury to be allowed to lecture to troops."

The following year, 1944, an informant heard Johnson speak to the Russia Today Society at the Red Lion pub in Chatham. He noted that up to 60 people attended the meeting, but was relieved that no soldier was present, at least not in uniform.

At the end of the war Johnson was awarded the Order of the Red Banner, in recognition of his "outstanding work as chairman of the joint committee for Soviet Aid", and in 1951 received the Stalin International Peace Prize.

Christopher Andrew, whose official history of MI5 is to be published in 2009 to mark the agency's 100th anniversary, described the Red Dean yesterday as "a one man front organisation" who "caused no damage whatever to national security".

Blank sheets in the files released yesterday show they have been heavily weeded, perhaps to protect informants. The files reveal that MI5 regularly tapped the phones of the Communist party of Great Britain and the Daily Worker, its newspaper.

The Red Dean never joined the CP. James Miller, alias folk singer Ewan MacColl, did. His MI5 file includes a 1955 press cutting from the Daily Worker advertising a celebration at the Albert Hall to mark the paper's 25th birthday. MacColl was billed with the Red Dean, and Harry Pollitt, the CP's general secretary.

MacColl and his first wife, Joan Littlewood - who became director at the Theatre Royal in Stratford, east London, were watched during the second world war when they lived in Hyde, Cheshire.